The present invention relates to telephone circuits which can be monolithically integrated and in particular to telephone interface circuits between the subscriber's telephone line and exchange control components which not only receive, amplify and transmit signals but also supply the line itself.
A serious problem which may arise with a subscriber's telephone line is the breakage of the line itself or of the telephone cable which contains it together with other lines.
If, following the breakage of the line, both the wires of the line are short-circuited to ground as is the case, for instance, when the line is cut on wet ground, the condition know as a grounded cable occurs.
In this condition the supply circuit supplies all the current available.
In telephone interface circuits, as is normally the case for all integrated circuits, there is a maximum instantaneous current limitation to avoid damage to the circuit components (the limitation current may have a typical value of 100mA).
The part of the interface circuit which acts as a supply source for the line operates, however, at high voltages (from -48 to -60 V with respect to the ground potential) and the electrical power dissipated is therefore very high.
Adequate limitation of the power dissipated must therefore be provided in the circuit by means of the heat protection conventionally incorporated in integrated circuits.
Although the problem can be partially resolved in this way in the case of an individual subscriber's telephone line, it is particularly serious when the grounded cable condition affects all of the telephone lines contained in a multiple telephone cable which has been accidentally cut.
The overall consumption of current may in this case exceed the current capacity of the exchange supply device (battery), causing the exchange protection apparatus to come into operation and thus completely closing down its operation.
Telephone communications, including those on lines not affected by the breakdown, are thus interrupted until the cable is repaired or isolated.
The only solution to the problem at present is to further limit the maximum current which can be supplied by the individual interface circuit, connected to a subscriber's telephone line, to a level compatible with the overall capacity of the exchange supply device, even in the case of a multiple grounded cable.
An excessive limitation of the current which can be supplied by the interface circuit to the subscriber's telephone line conflicts, however, with other requirements.